This invention relates generally to solid state force transducers in which a piezoresistive strain gage of semiconductive material is formed directly on a face of a force responsive member. For clarity, the invention will be described primarily as it relates to pressure transducers.
The patent literature describes many force transducers in which the force to be measured is applied to a single crystal of silicon, and the piezoresistive strain gage is formed by diffusing suitable impurities into selected areas of a face of the silicon crystal to alter the conductivity type of the silicon. The PN junction between the treated and untreated areas of the silicon crystal can then be reverse biased by applying suitable voltage to the strain gage, normally providing effective electrical insulation between the resistive elements of the gage and the surrounding silicon crystal.
However, since the breakdown voltage of a PN junction decreases progressively with increasing temperature, transducers depending for insulation upon such a reverse biased PN junction are inherently limited to operation at moderate temperatures. At room temperature the reverse leakage current is typically in the microampere range, and the breakdown voltage of the junction is typically from 100 to 120 volts. There is then an ample margin if the piezoresistive gage is operated at a moderate voltage such as 12 volts, for example. That margin decreases relatively slowly with increasing temperature up to about 80.degree. C. Above that temperature, however, the breakdown voltage decreases at a rapidly increasing rate, accompanied by increasing current leakage across the reverse biased PN junction. The maximum temperature for reliable and efficient operation is usually considered to be about 150.degree. C.
Prior art force transducers of the described type are also limited in operating temperature due to difficulties in mounting the silicon diaphragm or other force responsive element on a rigid support. No fully satisfactory procedure is available for bonding silicon to a support of stainless steel or the like in a manner which fully isolates the silicon from stresses due to temperature variations. Moreover, commonly used bonding materials, such as epoxides and soft solders, lose their beneficial properties at extreme temperatures. Thus, adhesives tend to melt or burn above about 200.degree. C. and to become so brittle as to lose all adhesion below about -60.degree. C. That problem is especially severe in the case of pressure transducers, since sealing materials tend to become porous or otherwise fail to maintain a hermetic seal at temperatures beyond approximately the same limits, permitting leakage around the pressure sensing diaphragm.
Some of those limitations in mounting pressure diaphragms are overcome to a considerable extent by the technique described in U.S. Pat. No. 3,417,361, which is assigned to the same assignee as the present application. In that technique the silicon diaphragm and its support structure essentially constitute portions of a unitary single crystal which encloses the pressure chamber. Pressure transducers made by that technique are capable of giving excellent results at conventional operating temperatures, fully justifying the additional care and expense required. However, they are still subject to temperature limitations due both to their dependence upon PN junctions for insulation and to the need for obtaining a hermetic seal between the silicon cell and the support hardware.
The temperature range of operation of conventional silicon force transducers may also be limited by the manner of connecting electrical leads to the silicon for bringing current to and from the strain gage. For example, many of the best silicon strain gages employ gold wires bonded to the silicon either directly or with a barrier layer such as nickel. Although such connections are highly satisfactory throughout the temperature range in which the gage itself can operate, the gold-silicon bond tends to fail above 360.degree. C., which is the eutectic temperature of those materials.